Marine instrument

ABSTRACT

A through-hull speed sensor of the magnetized paddlewheel sensor type wherein a paddlewheel is rotatably supported in a cavity adjacent a magnetic sensing device which generates electrical pulses corresponding to paddlewheel speed as the wheel rotates when the vessel moves through water. The sensor has improved low speed characteristics provided by a housing which extends the paddlewheel into the water below the hull. The bow of the housing provides a new leading edge, generating a smaller thickness boundary layer at the paddlewheel, so that the paddles project beyond the layer at low speeds. High speed performance is improved by having a high ratio of paddle fore-to-aft drag coefficient shape; keeping the periphery of the paddlewheel hub within, or flush with, the bottom of the cavity; a low paddle projection versus paddlewheel radius ratio; and by keeping the ratio of paddle cross-section versus available cavity cross-sectional space within an optimum range.

This is a continuation of co-pending application Ser. No. 007,527 filedon Jan., 28, 1987, now abandoned.

DESCRIPTION

1. Technical Field

This invention is in the field of marine instrumentation for providinginformation with respect to the speed of marine vehicles through water.

2. Background

U.S. Pat. No. 4,555,938 describes a transom mounted marine speed sensorof the paddlewheel type, in which the paddles or blades are asymmetricin shape and formed of magnetized amorphous magnetic material. As themarine vessel passes through water, the paddlewheel rotates about anaxis which is transverse the direction of travel. A Hall-effect deviceadjacent the paddlewheel senses the change in the magnetic fieldemanating from the paddles and generates an electrical signal directlyproportional to the rotational speed of the paddlewheel. Generally, therotational speed of the paddlewheel is linearly related to the vesselspeed, but this is not always the case; especially at the extremes ofhigh and low speeds.

The unenclosed design of the transom mounted paddlewheel sensor of the'938 patent has been found to improve linearity, at high speeds overthat obtainable in enclosed transom mounted paddlewheel sensors and thealternate enclosed "through-hull" type paddlewheel speed sensors, i.e.,sensors mounted through a hole formed in the vessel hull.

Various techniques have been devised, in an attempt to improve thehigh-speed performance of such enclosed paddlewheel speed sensors.Casani et al. in U.S. Pat. No. 3,531,988, suggest an open paddlewheelconstruction, in which four paddles are each supported by arms whichdefine an open space between the paddle and the wheel shaft. Maeder etal. in U.S. Pat. No. 3,457,782, employs a waterwheel having a pluralityof buckets, closed at their side, and recessed in the peripheral edge ofthe wheel. The slot within which the wheel rotates, is open on thedownstream side. According to Maeder et al., this provides a reliefspace which prevents a buildup of pressure on that side, which wouldvary the rotational characteristics of the wheel.

Despite the above efforts, and that of other workers in the art, a needexists for a through-hull speed sensor with improved performance,especially at high speeds, which does not at the same time sacrifice lowspeed linearity and accuracy.

DISCLOSURE OF THE INVENTION

Through a series of experiments, it has been determined that a number ofinterrelated factors affect the high and low speed performance ofenclosed cavity paddlewheel speed sensors. Chief among them are thefollowing:

I. Boundary Layer

I have found that the low speed characteristics of through-hullpaddlewheel sensors improves when the sensors are mounted in a housingwhich projects into the water a certain distance. While the exact natureof this phenomena as it relates to complex shapes is not fullyunderstood, it is believed that this improvement results from extensionof the paddles through the boundary layer.

A boundary layer of water is formed on the hull of boats as the boattraverses the water. The thickness of this layer varies principally withthe speed of the boat and the distance of the layer from the leadingedge of the water as it meets the bow or forward edge of the boat. Thefluid, i.e., water, within the boundary layer, attaches to the vesselhull and flows at a lesser rate relative to the boat as compared withthe water outside the layer. This rate varies throughout the thicknessof the layer.

For a given fixed paddlewheel sensor location, the boundary layerthickness δ at the sensor is directly proportional to the distance "X"between the sensor and the leading edge. If the sensor paddles do notproject beyond the thickness δ, true speed is not sensed.

For nearly all vessel speeds and operational conditions and practicalpaddlewheel mounting locations, the flow regime is turbulent. Therefore,the boundary layer thickness δ must be calculated using equations forturbulent flow conditions. Assuming a speed of 5 mph and a sensorlocated a distance X=10 feet from the leading edge, at that speed; theboundary layer thickness δ is calculated, for turbulent conditions, tobe about 2 inches. In contrast, at 55 mph, the boat rises, or planes,reducing the wetted surface and the leading edge moves back toward thesensor. Assuming the distance X is now about 2 feet; δ becomes about 3/8inches. It may thus be seen that a paddlewheel projecting a half-inchinto the water would not produce correct readings of true water speed atlow speeds, since it would not extend beyond the boundary layer.Furthermore, its accuracy would vary with speed, since at higher speeds,it projects a further distance into the boundary layer than at lowspeeds. While, theoretically, this problem could be solved by increasingpaddlewheel diameter, such an increase in diameter would require a muchlarger hole in the hull. Large diameter holes in hulls are to beavoided, for obvious reasons.

In accordance with the invention, a new leading edge is obtained a knownshort distance away from the speed sensor. This new leading edge may beconveniently obtained by providing a housing which projects below thehull. The projecting portion of the housing has a pointed bow portionextending upstream which forms the new leading edge. A pointed aftportion extends downstream. The projecting section of the housing ishorizontally aligned with the water surface by fairing blocks andprojects a distance about equal to or greater than the largestanticipated thickness δ of the new boundary layer formed by the newleading edge obtained at the bow of the projecting section at reasonableanticipated vessel speeds, i.e., about 3 miles per hour, or higher. Anintermediate portion of the projecting section houses the paddlewheel apredetermined, relatively short distance X' from the tip of the bowwithin a cavity formed therein.

In a preferred embodiment, the bow portion houses a sonar transducerelement for sensing depth of water; while the aft portion houses aHall-effect device for sensing the rotating magnetic field created asthe paddlewheel revolves. Optionally, a thermal sensor, in the form of athermistor, may be located in the aft or bow portion, to sense watertemperature.

With a paddlewheel rotating on an axis located about 3.75 inches fromthe point of the bow portion of the housing, δ is calculated to be about0.1 inches at 5 miles per hour and becomes progressively thinner asspeed is increased. Thus, the paddle can be made with a minimum amountof projection, as compared to a system without a projecting housing, thebow of which forms a new leading edge. Such a projecting sensor willproduce substantially accurate water speed signals at low speeds whileminimizing inaccuracy caused by changing boundary layer thickness atdifferent speeds.

II. Paddle Shape, Hub Placement, Paddle Projection, Cross-Section

The performance of the enclosed cavity through-hull paddlewheel sensors,especially at high speeds, i.e., above about 35 mph, has been found, inaccordance with the invention, to be related in a complex manner to fourmain factors:

(i) the forward versus rearward shape of the paddle;

(ii) the vertical placement of the hub, from which the paddles extend;

(iii) the relative projection of the paddles versus the diameter of thepaddlewheel; and

(iv) the ratio of paddle cross-section to available cavitycross-sectional space.

These factors are discussed below as follows:

(i) Paddle Shape: The forward shape should have a relatively largecoefficient of drag, as compared to the rearward shape, so as tominimize the force required to rotate the impeller within the cavity. Asspeed increases, i.e., at about 25-35 miles per hour, this force becomesgreater due to turbulence in the enclosed cavity. An acceptablefore-to-aft drag coefficient ratio is achieved in a preferred embodimentby forming the paddles in the shape of a half-hemisphere with the frontof the paddle, presenting a blunt, or bluff, frontal surface and theshape at the rear of each paddle is formed as a curvedhalf-hemispherical surface. This produces a frontal drag coefficient ofabout 1.17 and a rearward drag coefficient of about 0.42 (See "Elementsof Fluid Mechanics", D. G. Shepard ©1965, page 356) or a front-to-back(fore-to-aft drag coefficient ratio) ratio of about 2.8. A ratio of 2.5or more, is preferred.

(ii) Hub Placement: The paddlewheel should be vertically located withinthe cavity so that the periphery of the hub does not extend beyond thecavity. Pressure measurements, taken at several locations at the top ofthe paddlewheel cavity at various boat speeds, have shown that, as thehub is extended beyond the bottom of the cavity, negative pressurebuilds up within the cavity as speed is increased. The reason of thisphenomena may be caused, in part, by the extended hub which deflects thewater flow, creating an airfoil effect. I have found that by keeping thehub periphery level, or within the bottom of the cavity, reduces thistendency and increases the speed threshold before onset of cavitation.

(iii) Projection: In addition to the hub location, it is important thatthe Projection Ratio, i.e., the ratio of paddle length (paddleprojection), versus the overall radius of the paddlewheel, be keptrelatively low. This minimizes negative pressure buildup within thecavity at increased speeds. Increasing negative pressure leads,eventually, to cavitation within the cavity, with resultingunpredictable behavior of the rotating paddlewheel. In accordance withthe invention, a ratio of about 0.6, or less, has been found to besatisfactory for high speed operation.

(iv): Cross-Section: To further minimize build-up of negative cavitypressure and thus minimize the tendency for cavitation at higher speeds,the Cross-Section Ratio, i.e., the ratio of the cross-sectional shape ofthe paddles, taken in a plane transverse the direction of flow, versusthe available cross-sectional space in the cavity, in the same plane,should be optimized within a range about 0.25 to 0.5. In accordance withthe present invention, this Cross-Section Ratio is in the order of 0.40.This ratio is achieved by symmetrically contouring, or rounding-off, theside walls of the paddles, at the tips and symmetrically removingmaterial from the sidewalls as they join the hub. This reduces thenumerator of the ratio, i.e., paddle cross-section.

The factors involved in optimizing the Cross-Section Ratio are paddleshape and material and slow speed slippage. The paddle shape isconstrained by the requirement for a high fore-to-aft drag ratio, theneed to maximize paddle section near the outside diameter so as togenerate maximum rotational force (torque), and the need to havesufficient mass of amorphous magnetic material close to the cavity wallsso as to generate sufficient magnetic fluid strength 5 to switchcommonly available Hall-cell devices.

Reducing the cross-section ratio allows the entrained water in thecavity to be laterally displaced at higher paddlewheel rotational speedsand allows some venting of the negative pressure buildup in the cavity.

When the cross-sectional ratio drops below 0.25, linearity at slow speedis compromised, since bearing friction and viscous drag forces approachthe magnitude of applied rotational force and thereby causes increasedpaddlewheel slippage as the vessel travels through the water.

The lower limit to cross-sectional ratio is dependent on the requiredlinearity and accuracy of the paddlewheel at low boat speeds (5 milesper hour). As the frontal cross-sectional area of the paddle is reduced,the force applied to rotate the paddlewheel is proportionally reduced.While the profile drag may be somewhat reduced, due to the narrowersection, the viscous drag (i.e., frictional drag) and bearing frictionare not proportionally reduced. (The viscous drag remains nearlyconstant because the wetted surface of the paddlewheel is not reducedlinearly with reduction in paddlewheel cross-section.)

Most conventional prior art paddlewheels maintain a close tolerancebetween the cavity wall and the paddles and are analogous to positivedisplacement pumps whereby fluid is entrained between the paddles asthey rotate into the cavity and exhausted as the paddles exit. This nearpositive displacement design of most paddlewheels is a prime cause ofnegative pressure and eventual paddlewheel cavitation at higher vesselspeeds.

The above, and other features and advantages, will now be described, indetail, in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational frontal view of a high speed through-hull speedsensor in accordance with the invention in partial cut-away formincluding the hull and shim and fairing 13U and 13L, respectively.

FIG. 2 is a top view of the sensor of FIG. 1.

FIG. 3 is a cross-sectional view along the lines III-III of FIG. 1,which, for simplicity, omits the flanged nut 22 and the detailcross-section of transducer 60 and Hall-effect device 56.

FIG. 4 is an elevational plan view of paddlewheel 26 of the invention.

FIG. 5 is a side view of FIG. 4.

FIG. 6 is a cross-sectional view taken along lines VI--VI of FIG. 5.

FIG. 7 is an elevational view of an alternate embodiment speed sensor inaccordance with the invention with portions broken away.

FIG. 8 is a top view of FIG. 7.

FIG. 9 is a side view of FIG. 10.

FIG. 10 is an elevational plane view of a six-paddle paddlewheel used inthe FIG. 7 embodiment.

FIG. 11 is a cross-sectional view taken along lines XI--XI of FIG. 9.

BEST MODE OF CARRYING OUT THE INVENTION

The invention will now be described in detail in connection with thedrawings. Referring now to FIGS. 1-3, there is shown a preferredembodiment of the invention comprising a high speed through-hulltri-sensor device 10 for marine vessels. The device 10 may be mountedthrough the hull 11 of a boat (not shown). The sensor has a housingdesignated, generally, by the numeral 12, comprising a verticallyextending housing sleeve 12S, and lower body portion 12L, respectively.

The sleeve 12S is positioned in an opening through the hull 11. Upperwedge or shim and lower fairing block 13U and 13L, respectively, arewedged between the top surface of lower body portion 12L, hull 11 andlock-nut 22, to position sensor 10 in a vertical plane. Nut 22preferably has a bottom skirt, or flange portion 22F, which restsagainst the upper shim 13U and a hexagonal nut portion 22 with internalthreads, which engage a set of external threads 21 on the exteriorperiphery of the sleeve 12S.

A tubular body 52, the lower portion of which forms the paddlewheelcavity 30, is adapted to slideably insert into the sleeve 12S. Sealrings 32 and 34, respectively, are disposed in grooves on the peripheryof the body 52 and form a fluid-tight seal between the body and sleeve.Body 52 is of tubular shape and is formed of metal or plastic.

A cable 16 containing wires 19 is coupled through a bored hole 27 formedin an enlarged wall of the sleeve 12S. Wires 19 provide electricalconnection to components, such as Hall-effect device 56, sonartransducer 60, and thermistor 62, affixed to the inner walls of lowerbody portion 12L. The upper end of body 52 includes a tab 20, throughwhich a pull ring 18 is attached.

A second pull ring 14 is affixed to pin 17 which extends through a borein sleeve 12S and a transverse bore in body 52. The relative location ofthe bores assures that when pin 17 is in place, the body 52 isvertically aligned so that the bottom wall 52B is coplanar, or flush,with the bottom of the housing. Removable lock ring 15 is used to securethe pin 17 in place.

A paddlewheel 26 is rotatably mounted on axle 28 within the cavity 30formed in the lower portion of body 52. Paddlewheel 26 is an integralstructure having a hub 38 from which four asymmetric shaped paddlesextend about the periphery thereof. The axle 28 rotates within bearings37 disposed in a bore 39 which extends through a central opening in thehub 38, and into opposing recessed holes in the cavity side walls formedin body 52. The paddlewheel 26 is thereby rotatably suspended within thecavity 30. Preferably, paddlewheel 26 is formed of amorphous magnetizedmaterial, such as barium ferrite. The paddles 42 may be polarized withrespect to the hub, or with respect to each other. As the paddles rotateabout the shaft when the vessel traverses the water, the variation inmagnetic field is sensed by a Hall-effect device 56 mounted on aninterior wall 12I of housing 12.

The lower portion 12L of the housing extends about 1 inch, or more, (dueto the fairing block), into the water in operation, and thus may be morereadily subjected to impact from objects than prior art flush mounted,or low profile flanged, through-hull sensors. If the housing breaks,water may enter the boat through the opening. Therefore, the housingshould preferably be made of thick cast, non-corrosive metal.

Also, a preferred location for the Hall-effect device 56 is on a housingwall closely adjacent the cavity 30, such as wall 12I, where it can beencapsulated and protected from the water. Therefore, housing 12 shouldbe constructed of non-ferrous metal, so that it will be permeable to themagnet field emanating from the paddlewheel. Bronze is therefore apreferred material, since it meets all of the above criteria.

Sonic transducer assembly 60, thermal sensing device 62, and Hall-effectdevice 56, are substantially as described in U.S. Pat. No. 4,555,938,and are electrically coupled via wires 19 to depth indicator 90,temperature display 80, and speed display 70, respectively; also asdescribed therein. The interior of the lower housing, at the bow end12B, contains sensors 60 and 62 and is encapsulated in potting material64, such as polyurethane. Likewise, the interior at the aft end 12A,containing Hall-effect device 66, is encapsulated with polyurethane 64.

The upper wall 44 of cavity 30 is an arched surface, closely spaced fromthe tip T of the four paddles 42 as they rotate about axle 28. Theperiphery P of the hub 38 is approximately flush with the bottom surfaceB of the cavity. The profile of the paddles is contoured as a blufffrontal surface F and a convex curve aft surface A to produce a highdrag coefficient ratio (aft versus fore) of about 2.8. A highfore-to-aft ratio is produced when the aft drag coefficient is low andthe fore drag coefficient is high. In the embodiment shown, the foreshape is a flat, or near planar, surface and the high fore dragcoefficient of the paddle is calculated as about 1.17. The aft shape isa convex half-hemisphere producing a calculated low aft drag coefficientof about 0.42. This should be compared with the fore-to-aft ratio forthe asymmetric paddles of U.S. Pat. No. 4,555,938, which is calculatedto be about 1.5.

The radius of the paddlewheel 26, measured from tip to center, is about0.61 inches and the four paddles 42 project a distance of about 0.33inches from the cavity bottom B to produce a relatively low paddleprojection to paddlewheel radius ratio (Projection Ratio) of about 0.55.Known prior art paddlewheel sensors, such as that shown in U.S. Pat. No.4,555,938, have had Projection Ratios as high as 0.8. In accordance withthe invention, a ratio of 0.6, or less, is required for high speedperformance.

To further minimize build-up of negative cavity pressure and thusminimize the tendency to cavitate within the cavity at higher speeds,the cross-sectional shape of the paddles 42, taken in a plane transversethe direction of flow versus the available cross-sectioned space in thecavity in the same plane, i.e., Cross-Sectional Ratio, should be kept toa ratio of between 0.25 to 0.5. In accordance with the presentinvention, this ratio is in the order of 0.40. This ratio is achieved byusing a relatively thin cross-sectional paddle and by symmetricallyrounding off the side walls W (See FIG. 4) of the paddles at the tipsand symmetrically removing material from the sidewalls as they join thehub. This reduces the numerator of the ratio, i.e., paddlecross-section.

The paddlewheel diameter is about 1.22 inches and the hub diameter isabout 0.50 inches, resulting in a paddle length of about 0.36 inches.The paddle width is about 0.335 inches and the minimum gap between thepaddle tips T and the top of the cavity is about 0.030 inches. Thecross-sectional ratio is computed as follows:

Available Cavity Cross-Section=Cavity Width×Available Cavity Height=0.61inches X(0.36 inches+0.030 inches)=0.238 square inches.

The paddle profile can be modelled as a circle with a diameter of 0.335inches, which yields an area of about 0.088 square inches. Correctingfor area mainly at the root (base) of the paddle adds about 0.008 squareinches. Therefore, the cross-sectional ratio is: ##EQU1##

FIGS. 7 and 8 show an alternate embodiment of the invention, whereincorresponding parts carry the same numeral with a prime suffix. In thisembodiment, a low profile through-hull speed-only-sensor 10' isprovided, which mounts on a hull with the lower housing portion 12L'extending through the hull between wing nut 78 and lower flange 12F onhousing 12'. Tubular housing 12 may be formed of rigid plastic materialwith peripheral external threads 21' which engage with internal threadson cap 114 and nut 78. Within the housing, a tubular body 52' isslideably insertable with O-rings 32', forming a fluid-tight sealbetween housing 12' and body 52. Hall-effect device 56' is encapsulatedin body 52' within the roof or arch above cavity 30' closely adjacentpaddlewheel 26' rotatably mounted on axle 28' within cavity 30'.

Cable 16' is potted within body 52' and couples wires between theHall-effect device 56' and a speed meter (not shown). Tab 20' is formedon body 52' and pull ring 18' is disposed in a transverse hole on tab20' to enable body 52' to be removed from the housing 12'. The arrow Aindicates the bow direction in which the paddle would face when securedby cap 114. The cavity 30' and paddlewheel 26' may be appropriatelydimensioned, as described in the above referenced tri-sensor embodiment.

The housing projects outside the hull less than 1/4 inch, producing aminimum drag profile. Because the boundary layer becomes progressivelythicker at slower speeds, this embodiment sacrifices low speed accuracyfor minimum housing drag at high speeds.

FIGS. 9, 10 and 11 show the details of the paddlewheel 26', which hassix paddles 42' extending from the hub 38'. Six paddles are preferred insome applications because of the high possible pulse rate produced.Also, a six paddle device has less tendency to stall at low speeds. Thefore-to-aft ratio is about 2.8, as in the previous embodiment.

The Cross-Sectional Ratio of this embodiment is identical to theprevious embodiment, i.e., 0.40. The Projection Ratio is 0.45. Theperiphery P of hub 38' is located at, or above, the lower surface of thecavity 30'.

Equivalents

This completes the detailed description of the preferred embodiments ofthe invention. It will be apparent, however, to those skilled in theart, that many modifications and changes can be made in the embodimentswithout departing from the teachings and concept of the invention. It isintended that the following claims cover all equivalent modificationsand variations as fall within the scope of the invention.

What is claimed is:
 1. A marine speed sensor comprising:(a) a housinghaving an upper vertically extending portion and a lower horizontallyextending portion, said horizontally extending portion including an aft,a bow, and an intermediate section; (b) means for mounting the housingthrough an opening in the hull of a marine vehicle, such that the upperportion extends through the hull into the vessel and the lower portionprojects below the hull; (c) a paddlewheel having a plurality ofmagnetized paddles extending from a central hub, said paddlewheel beingmounted within a cavity formed within the lower portion of the housingfor rotation about an axle extending through said hub along an axistransverse an axis extending longitudinally between the bow and aftsections, the lowest portion of the periphery of said hub being locatedwithin the cavity and tangentially aligned with or vertically above thebottom surface of the lowest portion of said housing; and .Iadd.whereinthe width of the paddles on said paddlewheel between the walls of saidcavity are contoured such that the contoured width of said paddles issubstantially less than the width of the cavity and the cross-sectionalarea of the paddles in a plane transverse the direction of flow versusthe available cross-sectional area with the cavity is in a range betweenabout 0.25 to 0.50; and .Iaddend. (d) sensor means for sensingvariations in the magnetic field as the paddles rotate. .[.
 2. The speedsensor of claim 1 wherein the width of the paddle on said paddlewheelsbetween the walls of said cavity is contoured such that the contouredwidth of said paddles is substantially less than the width of the cavityand the cross-sectional area of the paddles in a plane transverse thedirection of flow versus the available cross-sectional area within thecavity is in a range between about 0.25 to 0.5..].
 3. The speed sensorof claim 1 wherein the sensor means is located adjacent said paddles andincluding coupling means for coupling signals sensed by said sensormeans to a speedometer.
 4. The speed sensor of claim 3 wherein saidsensor means is mounted on a wall of said horizontally extendingportion.
 5. The speed sensor of claim 4 wherein the paddlewheel ismounted in the intermediate section and the sensor means is mounted inthe aft section.
 6. The speed sensor of claim 5 including a sonartransducer element mounted in the bow section.
 7. The speed sensor ofclaim 6 including a thermistor mounted in the lower portion with thecoupling means, including means for coupling signals from saidtransducer element and thermistor to depth sounding and temperatureindicating instruments, respectively.
 8. The sensor of claim 1 whereinsaid cavity extends into the housing from a surface co-planar with thebottom surface of said housing.
 9. The sensor of claim 1 wherein saidcavity is formed in the lower portion of a tubular body inserted intosaid housing and retained by seal rings.
 10. A through-hull speed sensorfor mounting in an opening in the hull of a marine vessel comprising:(a)a housing secured in said opening; (b) a removable body disposed in saidhousing; (c) a paddlewheel cavity formed in said body; (d) a magnetizedpaddlewheel having a plurality of paddles extending from a central huband adapted to be rotatably mounted on an axle extending across saidcavity transverse the fore and aft direction of travel of the vessel;and wherein the ratio of the paddle length extending from the hub to theoverall radius of the paddlewheel is less than about 0.6; (e) a magneticsensor located adjacent said paddlewheel.
 11. A through-hull speedsensor for mounting in an opening in the hull of a marine vesselcomprising:(a) a housing secured in said opening; (b) a removable bodydisposed in said housing; (c) a paddlewheel cavity formed in said body;(d) a magnetized paddlewheel having a plurality of paddles extendingfrom a central hub and adapted to be rotatably mounted on an axleextending across said cavity transverse the fore and aft direction oftravel of the vessel; and wherein the ratio of the cross-section of thepaddles in a plane transverse the direction of flow versus the availablecross-sectional space within the cavity is in a range between about 0.25to 0.5; (e) a magnetic sensor located adjacent said paddlewheel.
 12. Thesensor of claim 11 wherein the ratio is in the order of 0.40.
 13. Thesensor of claim 11 wherein the side walls of the paddles are contouredinwardly to reduce the cross-section.
 14. A marine speed sensorcomprising:(a) a housing having an upper vertically extending portion,said horizontally extending portion including an aft, a bow, and anintermediate section; (b) means for mounting the housing through anopening in the hull of a marine vehicle, such that the upper portionextends through the hull into the vessel and the lower portion projectsbelow the hull; (c) a paddlewheel having a plurality of magnetizedpaddles extending from a central hub, said paddlewheel being mountedwithin a cavity formed within the lower portion of the housing forrotation about an axis transverse an axis extending longitudinallybetween the bow and aft sections, the periphery of said hub beinglocated adjacent or vertically above the lowest portion of said cavity;the width of the paddle on said paddlewheels between the walls of saidcavity being contoured such that the contoured width of said paddles issubstantially less than the width of the cavity and the cross-sectionalarea of the paddles in a plane transverse the direction of flow versusthe available cross-sectional area within the cavity is in a rangebetween about 0.25 to 0.5; and (d) sensor means for sensing variationsin the magnetic field as the paddles rotate.
 15. The speed sensor ofclaim 14 wherein said sensor means is mounted on a wall of saidhorizontally extending portion.
 16. The speed sensor of claim 15 whereinthe paddle wheel is mounted in the intermediate section and the sensormeans is mounted in the aft section.
 17. The speed sensor of claim 16including a sonar transducer element mounted in the bow section.
 18. Thespeed sensor of claim 17 including a thermistor mounted in the lowerportion with the coupling means, including means for coupling signalsfrom said transducer element and thermistor to depth sounding andtemperature indicating instruments, respectively.
 19. A through-hullspeed sensor for mounting in an opening in the hull of a marine vesselcomprising:(a) a housing secured in said opening; (b) a removable bodydisposed in said housing; (c) a paddlewheel cavity formed in said body;(d) a magnetized paddlewheel having a plurality of paddles extendingfrom a central hub and adapted to be rotatably mounted on an axleextending across said cavity transverse the fore and aft direction oftravel of the vessel; and wherein the ratio of the fore drag coefficientof the paddles to the aft drag coefficient of the paddles is about 2.5or more and the periphery of said hub does not extend below the lowestportion of said cavity; (e) a magnetic sensor located adjacent saidpaddlewheel.
 20. A through-hull speed sensor for mounting in an openingin the hull of a marine vessel comprising:(a) a housing secured in saidopening; (b) a removable body disposed in said housing; (c) apaddlewheel cavity found in said body; (d) a magnetized paddlewheelhaving a plurality of paddles extending from a circular central hub andadapted to be rotatably mounted on an axle extending through an openingin said hub across said cavity transverse the fore and aft direction oftravel of the vessel.Iadd.; the width of the paddles on said paddlewheelbetween the walls of said cavity being contoured such that the contouredwidth of said paddles is substantially less than the width of the cavityand the cross-sectional area of the paddles in a plane transverse thedirection of flow versus the available cross-sectional area within thecavity is in a range between about 0.25 to 0.50.Iaddend.; and whereinthe lowest point on the periphery of the hub is located tangentiallyadjacent to or vertically above the lowest portion of the cavity; (e) amagnetic sensor located adjacent said paddlewheel.